Typically, packers are used to isolate the annulus around the tubing from the zone to be perforated and for other applications where specific zones in the wellbore are to be isolated. "Spacing out" of the tubing at the wellhead in specifically the position desired has been a problem with designs used in the past. The reason the exact depth of placement of the sealing members is unknown is in the nature of the design of prior packers. Most of the prior designs have required the packer assembly to be lowered on a tubing string. The packer assembly was mounted on a mandrel which rode with the packer assembly until the desired depth is reached. In past designs, the release between the packer assembly and the mandrel has occurred by a combined motion requiring letting up on the tubing string or allowing it to come down, while at the same time applying a rotational force, with the result being a disengagement between the mandrel and the packing element support structure. The problem with this operation has been that the combined axial and rotational movement can result in an uncertainty as to when the necessary rotation to create the release has taken place.
The apparatus of the present invention addresses the problem of uncertainty of placement of the packer by providing a release mechanism between the mandrel and the packing element support structure which is actuated by a simple rotation without any axial movement of the tubing string to effect disengagement between the mandrel and the packing element support structure.
Another potential problem in known packers occurs when it is desired to set the packer at a fairly shallow depth from the surface. In those applications, the weight of the tubing string may be insufficient upon disengagement between the mandrel and the packing element support structure to place sufficient longitudinal force to be transmitted from the mandrel to the packing element support structure to sufficiently actuate the packing elements and the slips. The slips grip the wellbore to secure the position of the packing elements. The apparatus of the present invention addresses this problem by providing two alternative means to actuate the slips and the packing elements by relying in most cases on the weight of the tubing string to put sufficient force on the packing element support structure to actuate the slips and the packing elements. Alternatively, means are provided between the mandrel and the packing element support structure to actuate the slips and the packing elements by an upward pull on the tubing string rather than relying on letting up on the tubing string and using its weight to actuate the packer.
Another concern in packers of prior designs, especially those that have attempted to employ slips above and below the packing elements in a design featuring a removable option, has been the difficulty in getting the slips to let go of the casing when it is time to retrieve the packer. Related to this problem has been the concern of obtaining sufficient bite of the slips in to the casing or formation and ensuring that there isn't a loosening of the grip by the slips of the casing during operations. The apparatus of the present invention deals with these concerns by providing a locking mechanism to retain the necessary force on the slips and the packing elements, minimizing the possibility that they will let go in use. Additionally, the apparatus of the present invention is so configured that upon initiating the steps to retrieve the apparatus, the previously mentioned locking mechanism is defeated and initial movements of the apparatus components result in a release of the compressive force on the packing elements followed by a force applied to the slips to draw them inwardly to facilitate the release of the apparatus from the wellbore. For the purposes of brevity, all references in this application to wellbore are intended to include applications where the wellbore has a casing at the point of use of the apparatus as well as locations where the wellbore is uncased.
Some prior designs use an "L" or "J" slot to effectuate release between the packing element support structure and the mandrel, such as illustrated in U.S. Pat. No. 4,307,781. Even using this method generates some uncertainty as to the ultimate positioning of the packer since the gripping of the packing elements on the wellbore occurs during the pulling up procedure and there is some uncertainty as to the exact point at which the packing elements and slips will engage. The apparatus of the present invention has attempted to address the uncertainties of placement of the packer by providing a mechanism to effectuate the release of the packing element support structure from the mandrel with a simple rotation. Thereafter, initial slip engagement is preferably obtained prior to packing contact with the wellbore, thus the movement of the mandrel over a known distance creates the initial gripping of the slips and fixes the position of the apparatus, whereupon the packing element is engaged.